Nanoparticles

ABSTRACT

Methods for the preparation of polymer-templated core-shell nanoparticles include the steps of (a) preparing a cationic polymeric core material comprising polymeric micelles, and (b) coating the core material with a silica-comprising shell by depositing the shell onto the polymeric micelles from at least one silica precursor to form the core-shell nanoparticles. Compositions which include the core-shell nanoparticles are adapted to facilitate controlled delivery of at least one active agent into a system in response to controlled changes in the pH of the system.

This application is a divisional of commonly owned U.S. application Ser.No. 12/438,591, filed on 28 Aug. 2009 (now abandoned), which is the U.S.national phase of International Application No. PCT/EP2007/007729 filed5 Sep. 2007, which designated the US and claims priority to GreatBritain Application No. 0617480.9, filed 6 Sep. 2006, the entirecontents of each of which are hereby incorporated by reference.

The present invention is concerned with novel nanoparticles. Morespecifically, the invention relates to core-shell silica-copolymernanoparticles, methods for their preparation, and their potential uses.

There is growing academic and industrial interest in the synthesis andapplications of nanoparticles, most particularly nanoparticles having acore-shell structure in view of their potential use as delivery vehiclesfor active materials such as drugs. Consequently, much prior art isdevoted to the preparation of nano-sized particles of this type.

Specifically, several authors have considered the potential applicationsof core-shell nanoparticles comprising silica and, in this context,attention has been devoted to the synthesis of block copolymer-templatedsilica structures, and studies of their properties and possible uses.Moreover, the presence of core-forming materials which allowed for thepossibility of achieving triggered release of active materials from thecore of the particles could offer significant opportunities.

It is known that biomineralisation of silica, or biosilicification,occurs in water under ambient conditions for various biological systems,such as diatoms and sponges. Moreover, this natural process leads tohierarchical structures and multiple morphologies with precise nanoscalecontrol, features which continue to elude materials scientists. Ideally,any biomimetic approach to silica synthesis would be bothenvironmentally benign and controllable, in order to allow for thegeneration of a range of structures and morphologies.

Recent improvements in the understanding of biosilicification haveresulted in some studies which have successfully demonstrated silicaformation under ambient conditions.

Furthermore, it is well known that block copolymers can self-assembleinto a wide range of nanostructures that can be used for controlling theformation of various inorganic materials. However, blockcopolymer-mediated silica formation is seldom reported. Moreover, theproduction of such particles in a chemically efficient manner that allowfor morphological and structural control remains a major challenge.

Silica-based core-shell nanoparticles have been suggested for variousbioanalytical applications, such as drug delivery, bioimaging andbiolabeling. In such cases, the particles have been previouslysynthesised by coating functional cores with silica shells either byusing Stöber chemistry or by means of a microemulsion approach. Bothmethods do, however, require the use of non-ideal conditions, such aselevated temperatures, non-physiological pH values, and the presence oflarge amounts of surfactants and/or organic co-solvents.

It is apparent, therefore, that there is scope for the development ofalternative nano-sized particles, which may be obtained using convenientreaction conditions.

According to a first aspect of the present invention, there is provideda composition comprising core-shell nanoparticles, wherein saidnanoparticles comprise:

-   -   (a) cationic core material comprising polymer; and    -   (b) shell material comprising silica.

Preferably, the core material comprises copolymer micelles, morepreferably diblock copolymer micelles. Most preferably, said diblockcopolymer micelle has a core comprising at least one block of a firstpolymer and a corona comprising at least one block of a second polymer,wherein said second polymer is different to said first polymer.

Preferably, said copolymer comprises a first polymer and a secondpolymer which both comprise amino-based (alk)acrylate monomer units,more preferably tertiary amino-based (alk)acrylate units, mostpreferably tertiary aminoalkyl (alk)acrylate units. Particularlypreferably, said (alk)acrylate units comprise acrylate or, moreparticularly, methacrylate units.

In preferred embodiments, said tertiary aminoalkyl methacrylate unitscomprise dialkylaminoalkyl methacrylate units, especiallydialkylaminoethyl methacrylate units. In a particularly preferredembodiment, said copolymer comprises poly[2-(diisopropylamino)ethylmethacrylate)-block-2-(dimethylamino)ethyl methacrylate] (PDPA-PDMA).

According to the invention, said micelles may either be non-crosslinkedor shell crosslinked (SCL) micelles based on said polymers. Thus,especially preferred embodiments envisage non-crosslinked or shellcrosslinked micelles based on tertiary amine methacrylate-derived blockcopolymers such as poly[2-(diisopropylamino)ethylmethacrylate)-block-2-(dimethylamino)ethyl methacrylate].

The conventional synthetic route to shell crosslinked micelles involvescovalent stabilization of the micelle coronal chains, although polyioncrosslinking has also been recently suggested. However, there are noliterature reports of micelle shell cross-linking via biomineralization.

In the present invention, crosslinking of the micelles of said tertiaryamino-based (alk)acrylate copolymers is most conveniently achieved bypartially or fully quaternising the tertiary amino groups of saidcopolymers with bifunctional quaternising agents. Thus, in the case ofthe most preferred embodiment of the first aspect of the invention,partial crosslinking of poly[2-(diisopropylamino)ethylmethacrylate)-block-2-(dimethylamino)ethyl methacrylate] (PDPA-PDMA) maybe achieved by selective quaternisation/crosslinking of the PDMA chainswith a suitable bifunctional quaternising agent, for example abis(haloalkoxy)alkane, such as 1,2-bis-(iodoethoxy)ethane (BIEE). Inthis most preferred embodiment, the PDPA chains remain essentiallyunquaternised.

The invention also envisages analogous non-crosslinked quaternisedderivatives, wherein quaternisation is achieved by means ofmonofunctional quaternising agents, such as alkyl halides, in particularalkyl iodides such as iodomethane. However, it is believed that controlof the silica deposition process may be enhanced in the case ofcrosslinked materials.

The degree of polymerisation of the polymer is preferably controlledwithin specified limits. Thus, in the most preferred embodiment of theinvention, the degree of polymerisation of the PDPA-PDMA copolymer ispreferably controlled such that the mean degree of polymerisation of thePDPA falls in the range of 20-25 and the mean degree of polymerisationof the PDMA falls in the range of 65-70, with particularly favourableresults having been obtained with the PDPA₂₃-PDMA₆₈ copolymer, whereinthe subscripts denote the mean degrees of polymerisation of each block.In the said embodiment, PDPA units form the cores of the micelles andPDMA units form the coronas of the micelles.

Preferably, said shell material comprises silica which is deposed onsaid core material from at least one silica precursor. Optionally, saidat least one silica precursor may comprise an inorganic silicate, forexample an alkali metal silicate, such as sodium silicate. However,preferred silica precursors comprise organosilicate compounds,especially alkyl silicates such as tetramethyl orthosilicate ortetraethyl orthosilicate. Most preferably, said silica precursorcomprises tetramethyl orthosilicate. Said treatment is found toeffectively crosslink the copolymer chains in uncrosslinked micelles,and thereby stabilise the micelles towards dissociation.

Preferably, said nanoparticles have a particle size in the region offrom 10-100 nm, more preferably from 20-50 nm, most preferably from30-40 nm and, particularly preferably, the particle size is around 30nm.

Preferably the nanoparticles have an average specific size g (whereg=½×(length+width)) of about 300 nm or less. More preferably theparticles have an average size of about 200 nm or less. Even morepreferably the particles have an average size of about 100 nm or less.Preferably the particles have an average size of 1 nm or more. Morepreferably the particles have an average size of about 10 nm or more.

Preferably the average specific size of the void is 1 nm or more, morepreferably 3 nm or more, even more preferably 6 nm or more. Preferablythe average specific size of the void is 100 nm or less, more preferably80 nm or less, even more preferably 70 nm or less.

Preferably the shell is at least 1 nm thick, more preferably at least 5nm, even more preferably at least 10 nm. Preferably the shell is 75 nmthick or less, more preferably 50 nm or less, even more preferably 25 nmor less.

In a particular embodiment of the first aspect of the invention there isprovided a composition comprising core-shell nanoparticles, wherein saidnanoparticles comprise:

-   -   (a) cationic core material comprising a copolymer micelle; and    -   (b) shell material comprising silica        wherein said nanoparticles have an anisotropic rod-like        morphology. Preferably, in said embodiment of the invention,        said copolymer micelle comprises a diblock or triblock        copolymer.

According to a second aspect of the present invention, there is provideda method for the preparation of a composition comprising core-shellnanoparticles according to the first aspect of the invention, saidmethod comprising the steps of:

-   -   (a) preparing a cationic core material comprising polymer; and    -   (b) coating said core material with a shell comprising silica.

The polymeric core material may be prepared by any suitablepolymerisation technique, but particularly favourable results areachieved when employing methods such as group transfer polymerisationand controlled radical polymerisation. Said core material is then coatedwith silica by treatment with a suitable silica precursor.

The method according to the second aspect of the invention isparticularly suited to the preparation of the compositions comprisingcore-shell nanoparticles according to the more preferred and mostpreferred embodiments of the first aspect of the invention. Thus,particularly preferred embodiments envisage the preparation of cationicdiblock copolymers by sequential monomer addition using group transferpolymerisation of tertiary aminoalkyl methacrylates.

Full or partial quaternisation of said copolymers may be achieved by anyof the standard quaternisation techniques reported in the literature.Typically, therefore, treatment of said tertiary amino-based copolymerswith alkyl halides, most particularly alkyl iodides such as iodomethane,in suitable inert solvents facilitates the preparation ofnon-crosslinked quaternised derivatives, whilst crosslinked quaternisedcopolymers are obtained by treatment of the tertiary amino copolymerswith bifunctional quaternising agents such as bis(haloalkoxy)alkanes,for example 1,2-bis-(iodoethoxy)ethane, in appropriate inert solvents.Typically, said quaternisation reactions are carried out by treating thetertiary amino copolymers with quaternising agents at or around ambienttemperature (20-30° C.), preferably about 25° C., for a period of timeof between 1-100 hours, preferably between 24 and 72 hours.

Deposition of silica is carried out by simply treating the cationicpolymers with suitable silica precursors under mild conditions. Thus, inthe case of the preferred copolymer micelles, these materials may bestirred with a silica precursor, typically an organosilicate compound,especially an alkyl silicate such as tetraethyl orthosilicate or, mostpreferably, tetramethyl orthosilicate, for between 10 and 60 minutes at5-30° C. and a pH of between 6.2 and 9.0. In a typical reaction,PDPA-PDMA copolymer micelles may be treated with tetramethylorthosilicate for 20 minutes at 20° C. and pH 7.2. The method of thesecond aspect of the present invention does, in this regard, offersignificant advantages over the methods of the prior art, which requirethat silica deposition procedures should be carried out at low pHvalues, and typically at pH 1.

According to a third aspect of the present invention, there is provideda composition adapted to facilitate controlled delivery of at least oneactive agent into a system, said composition comprising core-shellnanoparticles according to the first aspect of the invention, whereinsaid composition is adapted to provide said controlled delivery inresponse to controlled changes in the pH of said system.

According to a fourth aspect of the present invention, there is provideda method for facilitating controlled delivery of at least one activeagent into a system, said method comprising introducing a compositionaccording to the third aspect of the invention into said system andchanging the pH of the system in a controlled manner so as to facilitatesaid delivery.

Preferred examples of said active agent include, for example, drugs,dyes and catalysts, and suitable systems into which they might bedelivered include such diverse examples as human and animal bodies,coatings and chemical reactors. In the case of the most preferredcompositions according to the first aspect of the invention, whereinsaid compositions comprise copolymers which comprise tertiaryamine-based alkyl (meth)acrylate units, controlled delivery of activeagents may be achieved by introducing said composition into a system andadjusting the pH of a system to a value of less than 6 by addition of asuitable acidic agent.

According to a further aspect of the present invention, there isprovided a thin-film coating comprising the present nanoparticles. Asused herein, “thin-film” refers to coatings having an average thicknessof 500 nm or less.

According to a further aspect of the present invention, there isprovided an optical coating comprising the present nanoparticles. Asused herein, the term “optical coatings” refers to coatings with anoptical function as major functionality. Examples of optical coatingsinclude those designed for anti-reflective, anti-glare, anti-dazzle,anti-static, EM-control (e.g. UV-control, solar-control, IR-control,RF-control etc.) functionalities. Preferably the present coatings havean anti-reflective functionality. More preferably the present coatingsare such that, when measured for one coated side at a wavelength between425 and 675 nm (the visible light region), the minimum reflection isabout 2% or less, preferably about 1.5% or less, more preferably about1% or less.

It will be apparent that it may be necessary to remove some or all ofthe core material from the particle in order to achieve some of thebenefits of the present particles. This may be achieved in any suitablemanner at any suitable point in the production process. Preferredmethods include, for example, thermodegradation, photodegradation,solvent washing, electron-beam, laser, catalytic decomposition, andcombinations thereof. Therefore, the scope of the present inventionencompasses core-shell nanoparticles where the core is present and wherethe core has been at least partially removed.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of the words, for example“comprising” and “comprises”, means “including but not limited to”, andis not intended to (and does not) exclude other moieties, additives,components, integers or steps.

Throughout the description and claims of this specification, thesingular encompasses the plural unless the context otherwise requires.In particular, where the indefinite article is used, the specificationis to be understood as contemplating plurality as well as singularity,unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties orgroups described in conjunction with a particular aspect, embodiment orexample of the invention are to be understood to be applicable to anyother aspect, embodiment or example described herein unless incompatibletherewith.

The invention will be described in further detail with particularreference to the accompanying drawings, in which:

FIG. 1 schematically shows the formation of core-shell silicananoparticles obtained by biomineralization of tetramethyl orthosilicate(TMOS) using either shell crosslinked (SCL) or non-crosslinked cationicblock copolymer micelles as templates. Both routes lead to well-defined,core-shell copolymer-silica nanoparticles. The use of non-crosslinkedmicelles, as shown in the upper route, additionally leads to in situsilica crosslinking.

FIG. 2 presents TEM images of copolymer-silica nanoparticles: (A)synthesised by directly using non-quaternised PDPA₂₃-PDMA₆₈ copolymermicelles as templates; and (B) formed using partially quaternisedcopolymer micelles (50% with respect to the PDMA shell); the inset in(B) is a typical high magnification image obtained after dispersing thesame particles directly into an acidic solution (pH 2). The scale barsare 100 nm.

FIG. 3 displays TEM images obtained for: (A) core-shell copolymer-silicananoparticles prepared by stirring a mixture containing 2.0 ml of a 0.25w/v % aqueous solution of partially quaternised shell crosslinkedmicelles [30% target degree of crosslinking for the PDMA chains]solution and 2.0 ml TMOS for 40 minutes (the top inset shows arepresentative hollow silica nanoparticle after pyrolysis of thecopolymer component by calcination at 800° C.; the lower insethighlights a typical core-shell particle); (B) core-shellcopolymer-silica nanoparticles formed using partially quaternised SCLmicelles (50% target degree of crosslinking with respect to the PDMAchains) using the same biomineralisation conditions as those employed in(A); (C) core-shell copolymer-silica nanoparticles formed 40 minutesafter stirring an initially homogeneous solution comprising 2.0 ml of a0.25 w/v % aqueous solution of partially quaternised SCL micelles [30%target degree of crosslinking for the PDMA chains], 2.0 ml TMOS and 2.0ml methanol; and (D) core-shell copolymer-silica nanoparticles formed120 minutes after stirring using the same conditions as described in(C). The scale bars are 50 nm in each case.

FIG. 4 shows the particle size distribution of the core-shellcopolymer-silica nanoparticles prepared from the PDPA₂₃-PDMA₆₈ copolymer(50% quaternised coronal PDMA chains using iodomethane) estimated fromthe TEM image shown in FIG. 2B. These particles have a TEMnumber-average diameter of 28±3 nm and an intensity-average diameter of34 nm, as judged from DLS measurements.

FIG. 5 shows a transmission electron micrograph of silica nanoparticlesobtained from micelle templates prepared using the quaternisedPDPA₂₃-PDMA₆₈ copolymer (100% quaternisation of the PDMA chains), usingbiomineralization conditions which were the same as those used fortemplating micelles prepared with the 50% quaternised copolymer; in thiscase there appears little or no evidence for the formation of core-shellcopolymer-silica nanoparticles, and silification appears to occurthroughout the micelle interior.

FIG. 6 shows a transmission electron micrograph of silica nanoparticles(the same particles as shown in FIG. 2B, formed by 50% quaternisedPDPA₂₃-PDMA₆₈ micelles) after dispersing in acidic solution at pH 2 withthe aid of an ultrasonic bath.

FIG. 7 illustrates ¹H NMR spectra of: (a) a molecular solution of thePDMA₆₈-PDPA₂₃ diblock copolymer (50% quaternised PDMA block usingiodomethane) in D₂O/DCl at pH 2 (signal G at δ 1.3-1.4 is due to thefour equivalent methyl groups of the protonated DPA residues); (b)micelles for the same copolymer obtained in D₂O at pH 7 (there is nolonger a G signal at δ 1.3-1.4 due to the DPA residues since the PDPAblock becomes deprotonated and forms hydrophobic micelle cores at thispH; (c) silica-coated nanoparticles derived from PDPA₂₃-PDMA₆₈ diblockcopolymer micelles (50% quaternised PDMA block) in D₂O at pH 2 (thesignal G at δ 1.3-1.4 corresponds to the protonated PDPA chains withinthe micelle cores); and (d) the same silica-coated nanoparticles in D₂Oat pH 7 (signal G at δ 1.3-1.4 disappears, indicating that the PDPAchains in the micelle cores become hydrophobic due to deprotonation).

FIG. 8 shows the TEM particle size distribution of the hybrid silicananoparticles (as shown in FIG. 3A; prepared using SCL micelles at atarget degree of crosslinking for the PDMA chains of 30%); thesecore-shell copolymer-silica nanoparticles have a number-average diameterof 32±5 nm and an intensity-average diameter of 35 nm from DLSmeasurements.

FIG. 9 shows Transmission Electron Micrographs of core-shellcopolymer-silica nanoparticles obtained by stirring a mixture containing2.0 ml of a 0.25 wt. % aqueous solution of partially quaternised (50%iodomethane-quaternised with respect to the PDMA shell) copolymermicelles and either (images A, B) 58 mg or (images C, D) 116 mg of TMOSat 20° C. for 20 minutes at pH 7.2.

FIG. 10 shows Transmission Electron Micrographs of core-shellcopolymer-silica nanoparticles obtained by stirring a mixture containing2.0 ml of a 0.25 wt. % aqueous solution of partially quaternisedcopolymer micelles (50% target degree of crosslinking with respect tothe PDMA shell, using BIEE for quaternisation) and either (images A, B)58 mg or (images C, D) 116 mg of TMOS at 20° C. for 20 minutes at pH7.2.

FIG. 11 shows TEM images taken after silica deposition usingPDPA₂₃-PDMA₆₈ diblock copolymer micelles with higher copolymerconcentrations, wherein copolymer-silica core-shell nanoparticles wereobtained by stirring a mixture containing 1.0 ml of either 1 wt. % or 2wt. % aqueous solutions of copolymer micelles 50% quaternised withiodomethane [with respect to the PDMA chains only] with either 116 mg or232 mg of TMOS at 20° C. for 20 minutes at pH 7.2, then diluting theparticles with 40 ml ethanol and centrifuging at 16,000 rpm for 30minutes, and finally redispersing in ethanol with the aid of anultrasonic bath. This centrifugation-redispersion cycle was repeated toensure removal of excess TMOS and unreacted silicic acid oligomers.

FIG. 12 presents TEM images from silica deposition processes withPDPA₂₃-PDMA₆₈ diblock copolymer micelles after much longer depositiontimes, wherein copolymer-silica core-shell nanoparticles were obtainedby stirring a mixture containing 2.0 ml of a 0.25 wt. % aqueous solutionof copolymer micelles 50% quaternised with iodomethane [with respect tothe PDMA chains] with 58 mg of TMOS at 20° C. for 8 hours at pH 7.2, andthen subjecting the particles to two ethanol washing and centrifugationcycles (16,000 rpm, 30 minutes).

FIG. 13 illustrates FT-IR spectra recorded for: (a) the precursorPDPA₂₃-PDMA₆₈ diblock copolymer; (b) copolymer-silica core-shellnanoparticles obtained after silica deposition onto shell crosslinkedmicelles obtained from the PDPA₂₃-PDMA₆₈ diblock copolymer (targetdegree of crosslinking=30% using BIEE) under the stated conditions (seeFIG. 3A); and (c) hollow silica nanoparticles obtained after pyrolysisof the copolymer by calcination at 800° C.; the FT-IR spectrum of thecopolymer-silica core-shell nanoparticles contains IR bands that arecharacteristic of both the silica network (1080 cm⁻¹, multipletcorresponding to Si—O stretching; 950 cm⁻¹, Si—OH vibration mode; 800cm⁻¹, Si—O—Si bending; 470 cm⁻¹, Si—O bending) and also the copolymer(the carbonyl ester stretch at 1730 cm⁻¹); this latter carbonyl banddisappears after calcination of the copolymer, as expected, suggestingthe formation of hollow silica particles, an eventuality which isconfirmed by the results of TEM studies.

FIG. 14 shows zeta potential vs. pH curves obtained for the original SCLmicelles prepared from the PDPA₂₃-PDMA₆₈ diblock copolymer at a targetdegree of crosslinking of 30% for the PDMA coronal chains (circles) andthe final copolymer-silica core-shell particles synthesised using amixture of 2.0 ml of a 0.25 wt. % SCL micelle solution (target degree ofcrosslinking=30%) and 2.0 ml TMOS for 40 minutes (squares); forcomparative purposes, the zeta potential curve obtained for an ultrafinecommercial 20 nm silica sol (Nyacol 2040) is also shown (triangles).

FIG. 15 shows the transmission electron micrograph of Au/silicananoparticles obtained by protonating the PDPA chains in the cores ofthe silica-coated micelles using HAuCl₄, followed by in situ reductionusing NaBH₄; this experiment confirms that the PDPA chains remainlocated within the micelle cores after silica deposition, as expected.

FIG. 16 schematically shows the synthesis of an ABC triblock copolymerbased on poly(ethylene oxide) (PEO), PDMA and PDPA, wherein aPEO₄₅-PDMA₂₉-PDPA₇₆ triblock copolymer was synthesized by Atom TransferRadical Polymerisation (ATRP) using a PEO-based macro-initiator(PEO₄₅-Br macro-initiator), via a PEO₄₅-PDMA₂₉ diblock copolymer.

FIG. 17 shows the ¹H NMR spectrum of the PEO₄₅-PDMA₂₉-PDPA₇₆ triblockcopolymer recorded in d₅-pyridine.

FIG. 18 presents TEM images for silica rods wherein silica depositionwas performed at 1.0% copolymer concentration, the resulting silica rodsbeing easily (re)dispersed by ultrasonication.

FIG. 19 illustrates comparative zeta potential vs. pH curves obtainedfor the original copolymer rods prepared from the PEO₄₅-PDMA₂₉-PDPA₇₆triblock copolymer (shown as squares), and the final silica rodssynthesised using a mixture of 1.0 ml of a 1.0 wt. % copolymer micellesolution and 0.20 g TMOS for 20 min (shown as triangles); forcomparative purposes, the zeta potential curve obtained for an ultrafinecommercial 20 nm silica sol (Nyacol 2040) is also shown (as circles).

Particularly favourable results have been achieved with compositionsbased on selectively quaternised non-crosslinked and shell crosslinkedmicelles derived from tertiary amine methacrylate-based blockcopolymers, a specific example being poly[2-(diisopropylamino)ethylmethacrylate)-block-2-(dimethylamino)ethyl methacrylate] (PDPA-PDMA),and such materials have proved to be particularly successful when usedas templates for the biomimetic formation of well-definedcopolymer-silica nanoparticles of less than 50 nm diameter. Diblockcopolymer micelles comprising either partially or fully quaternisedpoly(2-(dimethylamino)ethyl methacrylate) (PDMA) coronas and hydrophobicpoly(2-(diisopropylamino)ethyl methacrylate) (PDPA) cores in particularhave been used as nano-sized templates for the deposition of silica fromaqueous solution under mild conditions, i.e. at pH 7.2 and 20° C.

PDPA-PDMA diblock copolymers of this type are relatively easy tosynthesise over a range of block compositions and copolymer molecularweights using any suitable method such as group transfer polymerisationor controlled radical polymerisation. Such diblock copolymers dissolvemolecularly in acidic solution due to protonation of both polyamineblocks. On adjustment of the solution pH with aqueous base, micellarself-assembly occurs at around neutral pH; the deprotonated hydrophobicPDPA chains form the micelle cores and the cationic (protonated) PDMAchains form the micelle coronas. Alternatively, and depending upon theprecise block composition under investigation and the degree ofquaternisation, selected diblock copolymers can be dissolved directly inwater at around neutral pH to form well defined micelles.

Both non-crosslinked and SCL micelles of this type can be coated withsilica without loss of colloid stability. Silica deposition on the SCLmicelles is primarily confined to the cationic PDMA shell, leading tocore-shell copolymer-silica nanoparticles with pH-responsive PDPA cores.Moreover, in situ silica deposition effectively stabilises theuncrosslinked PDPA-PDMA micelles, which remain intact on lowering thesolution pH, whereas the original PDPA-PDMA micelles are found todissociate to give individual copolymer chains in acidic solution.

In a further embodiment of the invention, it has been shown that apoly(ethylene oxide)-PDMA-PDEA triblock copolymer facilitates thepreparation of highly anisotropic rod-like silica particles.

Shell crosslinking of these micelles can be readily achieved at highdilution using 1,2-bis-(2-iodoethoxy)ethane (BIEE) as a bifunctionalquaternising reagent under mild conditions. BIEE quaternises the PDMAchains selectively, leaving the much less reactive PDPA chainsuntouched.

The general approach to the preparation of the compositions according tothe first aspect of the invention is shown in FIG. 1, from which it willbe gleaned that the thickness of the deposited silica shell differsaccording to whether or not the copolymer micelle incorporatescrosslinking. The degree of quaternisation of the PDMA block can also bean important factor. The PDMA shell has significant cationic characterdue to either protonation and/or quaternisation, so it can act both as apolymeric catalyst and also as a physical scaffold for silica formation.Tetramethyl orthosilicate (TMOS) was employed as a silica precursor andbiomineralization was conducted in aqueous solution at 20° C. at aroundneutral pH.

Thus, in the first approach, a PDPA₂₃-PDMA₆₈ block copolymer is eitherpartially or fully quaternised by treatment with iodomethane intetrahydrofuran at 20° C. for 24 hours, and non-crosslinked micelles areformed by dissolution at pH 2 and adjustment of the pH to 7.2; finally,silica deposition occurs on treatment of the micelles with tetramethylorthosilicate for 10-40 minutes at room temperature and pH 7.2,resulting in the formation of silica crosslinked nanoparticles having arelatively thick silica shell when using a relatively large excess ofTMOS.

Alternatively, micelles are formed by dissolution of the PDPA₂₃-PDMA₆₈block copolymer at pH 2 and adjustment of the pH to 7.2, and themicelles are then shell crosslinked by quaternisation by treatment with1,2-bis-(2-iodoethoxy)ethane (BIEE) at 20° C. for 72 hours; silicadeposition is then carried out by treatment of the crosslinked micelleswith tetramethyl orthosilicate for 10-40 minutes at room temperature andpH 7.2, resulting in the formation of silica nanoparticles having arelatively thin silica shell when using a relatively large excess ofTMOS.

Initially, the present inventors carried out silica deposition usingnon-crosslinked micelles prepared directly from the PDPA₂₃-PDMA₆₈copolymer precursor as templates. Dynamic light scattering (DLS) studiesindicated an intensity-average diameter of 37 nm at 25° C. for thesemicelle templates. At pH 7.2 the PDMA chains in the micelle shell areapproximately 50% protonated, and therefore have appreciable cationiccharacter.²⁴

Silicification of the said micelles was achieved by mixing 2.0 ml of anaqueous micelle solution (0.25 w/v % at pH 7.2) with 1.0 ml tetramethylorthosilicate, and then stirring the initially heterogeneous solutionunder ambient conditions for 20 minutes. The silica-coated nanoparticlesthus obtained were washed with ethanol, then subjected to threecentrifugation/redispersion cycles at 16,000 rpm for 5 minutes.Redispersal of the sedimented nanoparticles was subsequently achievedwith the aid of an ultrasonic bath.

Thermogravimetric analyses of the product indicated that the meandiblock copolymer content of the silica nanoparticles was about 15% bymass. A typical Transmission Electron Micrograph (TEM) image obtainedfor these TMOS-treated micelles is shown in FIG. 2A. The formation oftemplated silica nanoparticles with core-shell structures is clearlyobserved, since the silica/PDMA hybrid shell is more electron-dense thanthe PDPA chains within the micelle cores. These nanoparticles have anumber-average diameter of around 35 nm, which is in reasonably goodagreement with the dimensions of the precursor micelles. However, inaddition to the formation of templated silica nanoparticles, someill-defined, non-templated silica structures are also observed in FIG.2A, indicating that the silica formation is not particularly wellcontrolled in this case. Ideally, silica formation should occurexclusively on the cationic copolymer micelles, rather than in bulksolution.

Improved control over silica deposition was, however, achieved whenemploying quaternised polymers. Initial trial experiments were conductedusing PDMA homopolymer, and it was found that on mixing 1.0 mltetramethyl orthosilicate and 1.0 ml aqueous PDMA homopolymer solution(concentration of DMA repeat units, [DMA]=0.064 M) at pH 7.2 and 20° C.,the initially heterogeneous solution became homogeneous after continuousstirring for 15 minutes (hydrolysis of TMOS, which produces silicicacid, allows the system to become homogeneous). By way of contrast, for50% and 100% quaternised PDMA homopolymers under identical conditions,the corresponding times required for the reaction solutions to becomehomogeneous were 25 minutes and 50 minutes, respectively. This suggeststhat quaternised PDMA chains catalyse slower, and therefore perhaps morecontrolled, hydrolysis of the TMOS precursor.

These experiments with PDMA homopolymer suggested that well-controlledsilica deposition might be achieved using partially or fully quaternisedPDPA₂₃-PDMA₆₈ copolymer micelles as templates. Thus, experiments wereconducted wherein selective quaternisation of DMA residues was achievedusing iodomethane under mild conditions. A 0.25 wt. % aqueous solutionof PDPA₂₃-PDMA₆₈ copolymer micelles in which the PDMA chains were 50%quaternised had an intensity-average diameter of 29 nm at pH 7.2, asindicated by Dynamic Light Scattering (DLS). Tetramethyl orthosilicate(1.0 ml) was added to 2.0 ml of the aqueous micelle solution at 20° C.,and silica deposition was allowed to continue for 20 minutes, withcontinuous stirring, prior to isolation via centrifugation.

TEM images of the purified core-shell copolymer-silica nanoparticlesobtained are shown in FIG. 2B. Core-shell nanostructures were clearlyobserved, with a number-average diameter of 28±3 nm. DLS studiesindicated an intensity-average diameter of 34 nm and a relatively narrowsize distribution, as illustrated in FIG. 4. In contrast to the resultsobtained for the non-quaternised diblock precursor, there was noevidence for non-templated silica structures in this case, suggestingthat secondary nucleation had been minimised.

TEM results obtained using micelles with 100% quaternised PDMA blocksare shown in FIG. 5, from which it is apparent that there is little orno evidence of the formation of a copolymer core, thus confirming thatpartially quaternised copolymers are a particularly preferred embodimentof the present invention. Thermogravimetric analyses, however, indicatedthat the mean diblock copolymer contents of the silica nanoparticlesderived from micelles with 50% and 100% quaternised PDMA blocks wereabout 18% and 16% by mass, respectively. Thus, quaternisation of thePDMA chains does appear to be beneficial for well-controlled silicadeposition. Moreover, these quaternised micelles produced hybridnanoparticles with much thicker, more well defined silica shellsrelative to those obtained using non-quaternised copolymer micelles (seeFIG. 2A) under the same biomineralisation conditions.

The present inventors have also established that the nanostructure ofthese copolymer-silica core-shell particles can be simply controlled bytuning the amount of TMOS used for silica deposition. Thus, for example,silica particles with thin shells and large copolymer cores wereobtained when using lower levels of TMOS. Well-defined silica particleswith a number-average diameter of around 26 nm (see FIG. 9A/9B) wereformed by stirring a mixture of 58 mg TMOS with 2 ml of a 0.25 w/v %solution of 50% quaternised copolymer micelles for 20 minutes. As shownin Table 1, thermogravimetric analysis of the product indicated that themean copolymer content of these core-shell copolymer-silica particleswas about 28% by mass, indicating a silica conversion of about 58%. Suchparticles have much thinner silica shells and larger copolymer cores.Moreover, colloidal stability was maintained even when the reaction timewas increased from 20 minutes to 8 hours when using this reduced amountof TMOS (see FIG. 12A/12B). The results obtained when increasing thequantity of TMOS in the above synthesis to 116 mg are shown in FIG.9C/9D. Again, there is no evidence for non-templated silica (such asthat observed in FIG. 2B), indicating efficient templating of thesesilica nanostructures. Further thermogravimetric analyses indicated thatthese core-shell copolymer-silica nanoparticles had lower copolymercontents (23%) compared to the core-shell copolymer-silica nanoparticlesshown in FIG. 9A/9C (28% copolymer content). This indicates that higherlevels of TMOS lead to more silica deposition under otherwise identicalconditions.

TEM studies provided further evidence of efficient micelle crosslinkingvia biomineralisation. As shown in FIG. 2B (see inset) and FIG. 6, thesilica crosslinked micelles retain their spherical core/shell structuresafter direct dispersion and drying at pH 2. ¹H NMR studies of thecore-shell copolymer-silica nanoparticles at pH 2 produced a signal at δ1.3-1.4 due to the protonated PDPA chains (see FIG. 7). When thesolution pH was increased to pH 7, however, this signal disappeared asthe PDPA chains became deprotonated and hence hydrophobic. Thus, thesespectroscopic studies confirmed that the PDPA chains in the micellecores are pH-responsive (i.e. they can become hydrophilic at low pH andhydrophobic at high pH), and this further illustrates the potential useof these new core-shell copolymer-silica nanoparticles inencapsulation/controlled release applications.

Typically, shell crosslinking is conducted at high dilution (normallyless than 0.5 wt. % copolymer micelles) in order to avoid inter-micellefusion. However, micelle crosslinking by biomimetic silica depositioncan be successfully performed at somewhat higher concentrations. Thus,as shown in FIG. 11A/11B, the mixing of 1 ml of a solution of 1.0 w/v %copolymer micelles (50% quaternised with respect to the PDMA shell) with116 mg TMOS for 20 minutes produced well-defined hybrid copolymer-silicacore-shell particles with a number average diameter of about 26 nm.Similar-sized particles were also obtained using 2.0 w/v % copolymermicelles (see FIG. 11C/11D). Thermogravimetric analyses (Table 1)indicated that the mean copolymer contents of the copolymer-silicacore-shell particles shown in FIGS. 11A/11B and 11C/11D were about 20and 22% by mass, respectively, indicating silica conversions of 87 and78%, respectively. Thus this biomimetic approach to SCL micelles bysilica deposition appears to be notably efficient, and to offerparticular advantages in terms of mild reaction conditions, fastreaction times and relatively inexpensive reagents when compared withthe methods of the prior art.

The present inventors also prepared SCL micelles by selectivequaternisation and crosslinking of the PDMA chains using1,2-bis-(2-iodoethoxy)ethane, and evaluated the resulting cationicmicelles as templates for silica deposition. The target degree ofcrosslinking for the PDMA coronal chains was 30 mol %. DLS studiesconducted at 25° C. indicated an intensity-average micelle diameter of37 nm for the precursor SCL micelles.

Biomineralisation was performed using tetramethyl orthosilicate underthe same conditions as those employed for non-crosslinked micelles. FIG.3A shows a typical TEM image of the resulting silica nanoparticles.Their intensity-average and number-average diameters from DLS and TEMare 35 nm and 32±5 nm (see FIG. 8), respectively, which are inreasonably good agreement with the values obtained for the SCL micelleprecursor. Furthermore, their core-shell structure is also clearlyevident. For example, the silica nanoparticle indicated by the lowerwhite square in FIG. 3A has a PDPA core of approximately 14 nm and asilica/PDMA hybrid shell thickness of around 11 nm. Biomineralisationstudies with SCL micelles prepared at a target degree of crosslinking of50% produced similar results, as shown in FIG. 3B. Compared to thesilica nanoparticles prepared using non-crosslinked micelles (FIG. 2A),the silica particles obtained from SCL micelle precursors have largercores and thinner shells. In addition, there is no evidence fornon-templated silica within the dispersion, indicating that silicadeposition is again well-controlled.

Silica deposition was also performed at lower levels of TMOS. Thus, onmixing a 2 ml aliquot of a 0.25 w/v % copolymer micelle solution (50%target degree of crosslinking using BIEE) with 58 mg TMOS for 20minutes, silica deposition led to aggregation, rather than a colloidallystable dispersion. TEM studies indicated the formation of core-shellsilica particles of about 17 nm, as well as interconnected, fusedprimary particles (see FIG. 10A/10B). Thermogravimetric analyses (seeTable 1) indicated a mean copolymer content of around 30% by mass,indicating a silica conversion of approximately 50%. The formation ofsilica nanoparticles was much improved by using a slight excess of TMOSunder the same conditions. Hence, mixing 2 ml of a 0.25 w/v % copolymermicelle solution (50% target degree of crosslinking using BIEE) with 116mg TMOS for 20 minutes produced a colloidally stable dispersion, asjudged by visual inspection. As shown in FIG. 10C/10D, hybridcopolymer-silica particles with a number-average diameter of about 20 nmwere obtained. Thermogravimetric analyses indicated a mean copolymercontent of about 24% by mass, indicating a silica conversion of around35%.

Silica deposition can be also controlled using SCL micelles underinitially homogeneous conditions. Thus, a 2.0 ml aliquot of a 0.25 wt. %SCL micelle solution was added to a mixture of 2.0 ml methanol and 2.0ml tetramethyl orthosilicate, wherein the methanol acted as a co-solventand ensured that the TMOS was miscible with the aqueous phase from thebeginning of the reaction. After continuing silica deposition for 40minutes, TEM studies of the obtained product, as illustrated in FIG. 3C,confirmed the expected formation of well-defined core-shellcopolymer-silica nanoparticles. Even after continuing the treatment for120 minutes, however, no evidence for non-templated silicananostructures was observed, as shown in FIG. 3D.

The SCL micelle-derived core-shell copolymer-silica nanoparticles shownin FIG. 3A were further characterised using thermogravimetric analyses,FT-IR spectroscopy and aqueous electrophoresis. Thermogravimetricanalyses indicated that the mean copolymer content of thecopolymer-silica particles was about 19% by mass, whilst the FT-IRstudies, illustrated in FIG. 13, confirmed silica formation, since bandswere observed at 1080, 950, 800 and 470 cm⁻¹ for these particles, due tothe presence of the inorganic component; these bands were found to beabsent in the spectra obtained for the copolymer micelles prior tobiomineralisation. After calcination at 800° C., the characteristicbands at 1726 cm⁻¹, associated with the pyrolysed copolymer, completelydisappeared, whilst those bands assigned to the thermally-stable silicawere still observed.

TEM studies indicated that the calcined copolymer-silica particlesbecame hollow silica particles after pyrolysis of the organic component.Zeta potential measurements also supported the deposition of silicawithin the coronal layer of the copolymer micelles, as shown in FIG. 14.The precursor SCL micelles (having a target degree of crosslinking forthe PDMA chains of 30%) had positive zeta potentials over the whole pHrange investigated, due to their cationic PDMA shells. However, thesilica-coated micelles exhibited negative zeta potentials over a wide pHrange, with an isoelectric point at around pH 3.3. This latter behaviouris similar to that found for aqueous colloidal silica sols (see FIG. 14)and is, therefore, consistent with the SCL micelles becoming coated witha silica overlayer.

The inventors also attempted the deposition of gold nanoparticles withinthese hybrid copolymer-silica particles. In order to achieve this,HAuCl₄ was initially used to protonate the weakly basic PDPA chainswithin the cores of the nanoparticles. Then, the AuCl₄ ⁻ counter-ionsassociated with the protonated PDPA chains were reduced in situ toproduce zero-valent gold nanoparticles, using NaBH₄ as a reducing agent.The colour of the copolymer-silica hybrid nanoparticles changed fromwhite to wine red after the reduction step, indicating the formation ofnano-sized gold sols. TEM observations, as illustrated in FIG. 15,provided evidence for the generation of gold sols within the cores ofthe copolymer-silica nanoparticles, although some disruption of thesilica shells was also apparent. The experiment also provided directevidence for the presence of the PDPA chains within the cores of thehybrid copolymer-silica particles.

Thus, the potential for encapsulation of other species, such as quantumdots or biologically-active molecules, is clearly illustrated. Indeed,as a consequence of their well-defined nanostructures, these hybridcopolymer-silica nanoparticles have potential applications inbiolabeling, biodiagnostics, targeted drug delivery, solubilization,catalysis and imaging, and as fillers and coatings.

The fact that mild conditions, fast reaction times, and accessiblereagents can be utilised herein may offer clear advantages whenpreparing commercially applicable processes. In addition, the ability tocontrol the size and/or properties of the particles offers benefits.

The use of silica also offers particular advantages in terms of thepotential applications of the materials of the invention. Thus, sincesilica is usually considered to be a ‘food-grade’ material, these newparticles have potential applications in food manufacturing.

It is clear from the work of the inventors that the effect of varyingthe degree of quaternisation and shell crosslinking of the diblockcopolymer templates under investigation has a significant effect on thenature of the silica nanoparticles that are produced during in situsilica biomineralisation, since either solid spheres (with no cavities),or structured core-shell spheres with thin shells, or structuredcore-shell spheres with thick shells can be obtained, depending on theprecise nature of the copolymer micelles.

The core-shell copolymer-silica nanoparticles of the present inventionare somewhat larger than those of the prior art (30 nm vs. 10 nm), andthis should allow higher loading capacities. The core-shell nature ofthe hybrid copolymer-silica particles has been clearly illustrated byTEM studies, and these results have been corroborated by small anglex-ray scattering studies (SAXS). The mean wall thicknesses obtained byTEM and SAXS are in good agreement.

Perhaps the most significant advantage of the present invention,however, lies in the fact that the core-forming PDPA block in theclaimed compositions is pH-responsive, and this offers the possibilityof pH-triggered release of hydrophobic actives from the cores of thehybrid copolymer-silica nanoparticles.

The use of ABC triblock copolymers has found particular success in thepreparation of predominantly anisotropic rod-like copolymer-silicaparticles, and the said nanorods should allow zero-order diffusionalrelease to be achieved. The synthesis of said nanorods is illustrated inFIG. 16 wherein a poly(ethylene oxide)-based macroinitiator (PEO₄₅-Br)is firstly reacted with 2-(dimethylamino)ethyl methacrylate (DMA) in thepresence of copper(I) chloride, then the product is further reacted with2-(diisopropylamino)ethyl methacrylate (DPA). The obtained copolymer wascharacterised by GPC and ¹H NMR, and the results are summarised in Table2 and FIG. 17, which shows the ¹H NMR spectrum of the triblock copolymerrecorded in d₅-pyridine.

This copolymer was designed to self-assemble into colloidal micellaraggregates with PDPA cores, PEO coronas and PDMA inner shells. Since thePDMA block has a pK_(a) of around 7.0, these residues are approximately50% protonated at pH 7.2. Thus, silica deposition was expected to occurexclusively within the cationic PDMA inner shells, with the coronal PEOblocks imparting steric stabilization. Thus, it is believed that silicadeposition can be performed at relatively high copolymer concentrationswithout inducing particle fusion.

Silica deposition was performed at 1.0% copolymer concentration toproduce the anisotropic rod-like copolymer-silica particles, which wereeasily (re)dispersed by ultrasonication. The resulting silica rods werecharacterized using TEM, thermogravimetric analyses, FT-IR spectroscopyand zeta potential measurements. FIG. 18 shows a representative TEMimage of the silica rods. FT-IR studies confirmed silica formation andpolymer encapsulation, since bands were observed at 1080, 950, 800 and470 cm⁻¹ due to the inorganic component, and at 1726 cm⁻¹ due to thecarbonyl ester stretch of polymer for these silica rods.Thermogravimetric analyses indicated that the mean copolymer content ofthese hollow silica rods was about 26% by mass and, as shown in FIG. 19,zeta potential measurements indicated the successful coating of silicaonto the copolymer micelles.

The invention will now be further illustrated, though without in any waylimiting the scope of the disclosure, by reference to the followingexamples.

EXAMPLES Example 1

PDPA₂₃-PDMA₆₈ diblock copolymer was synthesised by sequential monomeraddition using group transfer polymerisation according to Chem. Commun.1997, 671-672. Gel permeation chromatography analysis indicated an M_(n)of 18,000 and an M_(w)/M_(n) of 1.08 using a series of near-monodispersepoly(methyl methacrylate) calibration standards. The mean degrees ofpolymerisation of the PDPA and PDMA blocks were estimated to be 23 and68, respectively, using ¹H NMR spectroscopy.

Non-crosslinked micelles of the PDPA₂₃-PDMA₆₈ diblock copolymer (degreeof quaternisation=0%) were prepared by molecular dissolution at pH 2,followed by adjusting the solution pH to pH 7.2 using NaOH. Dynamiclight scattering (DLS) studies at 25° C. indicated an intensity-averagemicelle diameter of 37 nm for a 0.25 wt. % copolymer micelle solution atpH 7.2.

Silicification of the said micelles was achieved by mixing 2.0 ml of anaqueous micelle solution (0.25 w/v % at pH 7.2) with 1.0 ml tetramethylorthosilicate, and then stirring the initially heterogeneous solutionunder ambient conditions for 20 minutes. The hybrid core-shellcopolymer-silica nanoparticles thus obtained were washed with ethanol,then subjected to three centrifugation/redispersion cycles at 16,000 rpmfor 5 minutes. Redispersal of the sedimented core-shell copolymer-silicananoparticles was subsequently achieved with the aid of an ultrasonicbath.

Example 2

PDPA₂₃-PDMA₆₈ diblock copolymer was synthesised by sequential monomeraddition using group transfer polymerisation as in Example 1.

Partial quaternisation of the PDMA block (targeting a degree ofquaternisation of either 50% or 100%) using iodomethane was conducted inTHF for 24 hours, as described in Macromolecules 2001, 34, 1148-1159.

Non-crosslinked micelles prepared using either 50% or 100% quaternisedPDPA₂₃-PDMA₆₈ diblock copolymers were also prepared by pH adjustment, asdescribed in Example 1. DLS studies conducted at pH 7.2 indicatedintensity-average diameters of 29 nm and 26 nm for 0.25 wt. % aqueoussolutions of 50% and 100% quaternised copolymer micelles, respectively.

Tetramethyl orthosilicate (1.0 ml) was added at 20° C. to 2.0 ml of a0.25 wt. % aqueous solution of PDPA₂₃-PDMA₆₈ copolymer micelles in whichthe PDMA chains were 50% quaternised, and silica deposition was allowedto continue for 20 minutes, with continuous stirring, prior to isolationvia centrifugation.

DLS studies on the hybrid core-shell copolymer-silica nanoparticlesobtained using the 50% quaternised copolymer precursor indicated anintensity-average micelle diameter of 34 nm at around pH 7.

Example 3

PDPA₂₃-PDMA₆₈ diblock copolymer was synthesised by sequential monomeraddition using group transfer polymerisation, and non-crosslinkedmicelles of the PDPA₂₃-PDMA₆₈ diblock copolymer were prepared asdescribed in Example 1.

Shell crosslinking of the coronal PDMA chains was achieved by adding abifunctional quaternising agent, 1,2-bis-(2-iodoethoxy)ethane (BIEE,0.15 moles per DMA residue for a target degree of cross-linking of 30%)to a 0.25% PDPA₂₃-PDMA₆₈ copolymer micelle solution at pH 7.2. Shellcrosslinking was carried out at 25° C. for at least 72 hours. Aftershell crosslinking, DLS studies indicated an intensity-average diameterof 32 nm and TEM studies suggested a number-average diameter of 26 nmfor the dried SCL micelles. On adjusting the aqueous SCL micellesolution to pH 2, DLS studies indicated an intensity-average diameter of45 nm due to swelling of the SCL micelles.

This DLS experiment also confirmed successful shell crosslinking, sincethe non-crosslinked micelles simply dissociate at low pH to form amolecular solution, because the PDPA chains are highly protonated, andhence no longer hydrophobic, at low pH. In addition, SCL micellesprepared using the 50% quaternised copolymer had an intensity-averagediameter of 37 nm at pH 7.2 as indicated by DLS.

Silica deposition was achieved by adding a 2.0 ml aliquot of a 0.25 wt.% SCL micelle solution to a mixture of 2.0 ml methanol and 2.0 mltetramethyl orthosilicate, wherein the methanol acted as a co-solventand ensured that the TMOS was miscible with the aqueous phase. Aftercontinuing silica deposition for 40 minutes, TEM studies of the obtainedproduct confirmed the formation of well-defined core-shellcopolymer-silica nanoparticles, as illustrated in FIG. 3C. Even aftercontinuing the treatment for 120 minutes, however, no evidence fornon-templated silica nanostructures was observed, as shown in FIG. 3D.

Example 4

PEO₄₅-PDMA₂₉-PDPA₇₆ triblock copolymer was synthesized by Atom TransferRadical Polymerisation using a PEO-based macro-initiator by firstlyadding the macro-initiator (1.00 g, 0.463 mmol) to a 25 ml one-neckflask, then degassing by three vacuum/nitrogen cycles, followed by theaddition of DMA (2.18 g, 13.88 mmol, target DP 30), 2,2′-bipyridine(144.5 mg, 0.925 mmol) and then 3.2 ml of a degassed 95/5 v/v IPA/watermixture. The solution was placed in a 40° C. oil bath and stirred untilhomogeneous. Copper(I) chloride (45.8 mg, 0.463 mmol) was then added andthe reaction was carried out at 40° C. for 3.5 hours under nitrogen withcontinual stirring. After this time, the DMA monomer conversion reached96%, as determined by ¹H NMR spectroscopy.

Thereafter, a mixture of DPA (4.94 g, 23.13 mmol, target DP=50) and 5.0ml of a 95/5 v/v IPA/water mixture was added. The second-stagepolymerization was carried out at 40° C. for 18.5 hours, before beingterminated by exposure to air. ¹H NMR analysis showed that the DPAmonomer conversion reached 99%. The copolymer solution was diluted withTHF (200 ml) and passed through a silica column to remove the spentcatalyst. The copolymer solution was then concentrated under vacuum andthe solid copolymer was precipitated into deionized water (100 ml) toremove residual monomer and any unreacted PEO-DMA diblock copolymer. Thepurified white copolymer was isolated by freeze-drying under vacuumovernight to give an overall yield of 6.1 g (76%).

The micellar rods formed by the PEO₄₅-PDMA₂₉-PDPA₇₆ triblock copolymerwere prepared by molecular dissolution at pH 2, followed by adjustingthe solution pH to 7.2 using NaOH. The final copolymer concentration was1.0 wt. %. Silica deposition was achieved by adding excess TMOS (0.20 g;i.e. a TMOS:copolymer mass ratio of 20:1) to 1.0 ml of copolymersolution and silicification was then conducted for 20 minutes at 20° C.and pH 7.2. Silica rods were obtained by washing with ethanol, followedby three centrifugation/redispersion cycles at 13,000 rpm for 15minutes.

TABLE 1 TGA results of silica synthesized using the PDPA₂₃-PDMA₆₈diblock copolymer micelles under various quaternisation conditions at20° C. and pH 7.2. Precursor micelles Mel(50) Mel(50) Mel(50) BIEE(50)BIEE(50) BIEE(50) Mel(50) Mel(50) Concentrations/ 0.25 0.25 0.25 0.250.25 0.25 1.0 2.0 wt. % Copolymer/mg 5 5 5 5 5 5 10 20 TMOS/mg 58 1161000 58 116 1000 116 232 Target polymer content from reaction 18 36 1.318 36 1.3 18 18 feeding/wt. % Actual polymer content from TGA/wt. % 2823 18 30 24 19 20 22 Silica Conversion/% 56 36 6 51 34 5 87 78 Diametersfrom TEM (nm) 33 33 28 20 23 26 33 35

TABLE 2 Summary of molecular weight data obtained for the PEO₄₅-Brmacro-initiator, PEO₄₅-PDMA₂₉ diblock precursor and the finalPEO₄₅-PDMA₂₉-PDPA₇₆ triblock copolymer. AB diblock ABC triblock ABCTriblock composition A block Conversion Conversion Targeted MorphologiesCalculation M_(n) M_(w)/M_(n) of DMA M_(n) M_(w)/M_(n) of DPA M_(n)M_(w)/M_(n) Rods PEO₄₅-DMA₂₉-DPA₇₆ 3,100 1.08 96 8,400 1.18 99 19,5001.20

The invention claimed is:
 1. A method for the preparation ofpolymer-templated core-shell nanoparticles comprising the steps of: (a)preparing a cationic polymeric core material comprising polymericmicelles employing a quaternized polymer comprising the steps of (i)preparing diblock copolymer micelles comprisingpoly[2-(diisopropylamino)ethylmethacrylate)-block-2-(dimethylamino)ethyl methacrylate]copolymer(PDPA-PDMA); (ii) partially or fully quaternizing the tertiary aminogroups of 2-(dimethylamino)ethyl methacrylate; and (b) coating said corematerial with a shell comprising silica by depositing the shell onto thepolymeric micelles from at least one silica precursor by stirring thecore material with a silica precursor for between 10 and 60 minutes at 5to 30° C. and a pH of between 6.2 and 9.0 to form the core-shellnanoparticles comprising silica shell having thickness of at least 5 nm.2. The method as in claim 1, wherein step (a) is practiced by preparingthe polymeric core material by group transfer polymerisation orcontrolled radical polymerisation.
 3. The method of claim 1, whereinstep (a) is practiced by controlling the degree of polymerisation of thePDPA-PDMA copolymer is controlled such that the mean degree ofpolymerisation of the at least one PDPA block falls in the range of20-25.
 4. A composition as claimed in claim 3, wherein the degree ofpolymerisation of the PDPA-PDMA copolymer is such that the mean degreeof polymerisation of the at least one PDMA block falls in the range of65-70.
 5. The method of claim 1, wherein the nanoparticles formed bystep (b) have an average specific size (g) of about 300 nm or less. 6.The method of claim 1, wherein the nanoparticles formed by step (b) havean average particle size is in the region of from 10-100 nm.
 7. Themethod of claim 1, wherein the nanoparticles formed by step (b) have ananisotropic rod-like morphology.